1. Field of the Invention
The present invention relates to a rotation angle detector for detecting the rotation angle of a rotary shaft.
2. Description of the Related Art
In machine tools or robots, a rotation angle detector is attached to, for example, an output shaft of a drive motor, in order to detect the actual position of a tool or hand. As such a rotation angle detector, an optical detector or a magnetic detector has been known. The rotation angle detector comprises a rotating body. The rotating body has track parts in which slits or magnetic elements are circumferentially arranged. The rotation angle detector can detect the rotation angle of a rotary shaft by detecting varying signals using the track parts.
In conventional arts, devices for detecting failures of a rotation angle detector have been known. For example, Japanese Unexamined Patent Publication No. 2010-266260 discloses an abnormality monitoring device which determines that there is a failure when the amplitudes of a plurality of signals fall below a predetermined threshold value. Further, Japanese Unexamined Patent Publication No. 2005-147733 discloses an abnormality detecting device which determines that there is a failure when the voltage values of signals of different phases are identical, and electrical angles are different from the previously stored normal electrical angle.
In the devices disclosed in these publications, abnormalities can be detected only after normal operations cannot be performed due to, for example, breaking of wires. If parts necessary for repair are ordered after a rotation angle detector fails to operate properly, there is a problem that a long time may be taken to restore the rotation angle detector. Alternatively, the parts necessary for repair may be store beforehand. However, there is a problem that a large amount of effort may be required to manage the parts, or a space for storage of a lot of parts for repair may be necessary.
On the other hand, Japanese Unexamined Patent Publication No. 06-66594 discloses a position detector in which a plurality of threshold values are set for the amplitudes of a plurality of signals so as to detect an abnormality. The position detector can predict the possibility of failures before the position detector fails to operate properly.
It is preferable that failures of a rotation angle detector are quickly detected. Examples of the causes of failures of the rotation angle detector include breaking of cables, electrical noise, and entry of foreign substances.
In conventional arts, it is difficult to identify these causes of failures. Thus, it is difficult to properly perform a repair after a complete failure is achieved. For example, it is difficult to identify the cause of a failure, and accordingly, the failure may occur again after the rotation angle detector is repaired. It is preferable that the possibility of a failure can be detected in early stages in order to identify the cause of the failure.
In particular, examples of foreign substances which may enter a rotation angle detector include liquid foreign substances, such as lubrication oil, cutting fluid, etc. and solid foreign substances, such as chips etc. A failure caused by entry of foreign substances into a rotation angle detector can cause a major failure, such as corrosion of parts caused by cutting fluid or breakage of parts caused by chips. Thus, it is preferable that the entry of foreign substances into a rotation angle detector can be detected in early stages.